canvas: fix bugs on HTMLCanvasElement::copiedImage()

Source/core/html/HTMLCanvasElement.cpp

Index: Source/core/html/HTMLCanvasElement.cpp
diff --git a/Source/core/html/HTMLCanvasElement.cpp b/Source/core/html/HTMLCanvasElement.cpp
index 05e2a22819dca4e956262e159244bbb40eaf5844..e6cafc4689bd5d60fc14aca75efc3a4c280667e9 100644
--- a/Source/core/html/HTMLCanvasElement.cpp
+++ b/Source/core/html/HTMLCanvasElement.cpp
@@ -706,9 +706,14 @@ PassRefPtr<Image> HTMLCanvasElement::copiedImage(SourceDrawingBuffer sourceBuffe
     if (!m_context)
         return createTransparentImage(size());
 
-    if (!m_copiedImage && buffer()) {
-        if (m_context && m_context->is3d())
-            m_context->paintRenderingResultsToCanvas(sourceBuffer);
+    // The concept of BackBuffer is valid on only WebGL.
+    if (m_context->is2d())

[aandrey, 2014/12/08 20:36:46]

Seems like this code has no effect (unless a context can be both 2d and 3d).

[dshwang, 2014/12/08 20:46:25]

That's right. It has no effect, because
CanvasRenderingContext2D::paintRenderingResultsToCanvas does nothing. I add it
for documentation purpose.

+        sourceBuffer = FrontBuffer;
+
+    bool needToUpdate = !m_copiedImage;
+    if (m_context->is3d())
+        needToUpdate |= m_context->paintRenderingResultsToCanvas(sourceBuffer);

[dshwang, 2014/12/08 20:17:08]

That's good question! After re-thinking, vector is not needed because this line
lets HTMLCanvasElement::copiedImage know if it's needed to update m_copiedImage.

One thing to note is that m_copiedImage exists to avoid repeated image creation
in one frame, and code without Vector must create image every time if clients
interleave to call BackBuffer and FrontBuffer although it's very hard on current
code base. 

+    if (needToUpdate && buffer()) {
         m_copiedImage = buffer()->copyImage(CopyBackingStore, Unscaled);
         updateExternallyAllocatedMemory();
     }